The walnut twig beetle (WTB), Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), is associated with thousand cankers disease of walnut (Juglans sp.). Spores of a newly identified fungus, Geosmithia morbida, are introduced into the phloem when the beetles bore through the bark. (See Kola{hacek over (r)}ik, M., Freeland, E., Utley, C., and Tisserat, N. 2011. Geosmithia morbida sp. nov., a new phytopathogenic species living in symbiosis with the walnut twig beetle (Pityophthorus juglandis) on Juglans in USA. Mycologia 103: 325-332.) The infection results in numerous cankers that eventually kill the tree, often within 3 years of the first appearance of symptoms.
Thousand cankers disease is a walnut disease of national significance that threatens the health and longevity of a variety of walnut species and their rootstocks, including walnuts of the edible crop (Juglans regia) industry and walnuts of the timber (Juglans nigra) industry. (See Tisserat, N., Crenshaw, W., Leatherman, D., Utley, C., and Alexander, K. 2009. Black walnut mortality in Colorado caused by the walnut twig beetle and thousand cankers disease. Plant Health Progress. Published 11 August 2009; Seybold, S. J., Haugen, D., O'Brien, J., and Graves, A. D. 2011. Thousand cankers disease. USDA Forest Service, Northeastern Area State and Private Forestry Pest Alert, NA-PR-02-10, May, 2010, reprinted August 2010 and October 2011, 2 pp.; Tisserat N., Cranshaw W., Putnam, M., Pscheidt, J. Leslie, C. A., Murray, M., Hoffman, J., Barkely, Y., Alexander, K., and Seybold, S. J. 2011. Thousand cankers disease is widespread on black walnut, Juglans nigra, in the western United States. Online. Plant Health Progr. (Published Jun. 30, 2011) doi:10.1094/PHP-2009-0811-01-RS.). Notably, one of the most susceptible species, Juglans nigra is an economically valuable hardwood tree species in North America, with a standing timber value in the range of $500 billion. The WTB, vector of the fungus causing the disease, is expanding its range and causing increased tree mortality in the western U.S. and was recently discovered near Knoxville, Tenn. (July 2010), Richmond, Va. (July 2011), and Bucks Co., Pa. (August 2011). (See Seybold, S. J., Coleman, T. W., Dallara, P. L., Dart, N. L., Graves, A. D., Pederson, L., and Spichiger, S.-E. 2012a. Recent collecting reveals new state records and the extremes in the distribution of the walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), in the United States. Pan-Pacific Entomol. (accepted, in press May 30, 2012) and Seybold, S. J., Haugen, D., O'Brien, J., and Graves, A. D. 2011. Thousand cankers disease. USDA Forest Service, Northeastern Area State and Private Forestry Pest Alert, NA-PR-02-10, May 2010, reprinted August 2010 and October 2011, 2 pp.) Northern California black walnut, Juglans hindsii, is rapidly succumbing to this threat in California. Id. Unfortunately, little is known about the semiochemicals that may mediate the colonization of host trees by WTB. Moreover, WTB is an unusual species in that it is an angiosperm-infesting Pityophthorus, as compared to other twig beetles, which infest conifers, and in that it colonizes the largest branches and main stem of the tree, whereas other species are confined to the twigs. Few products or methods exist for detecting and combating thousand cankers disease of walnut and none have demonstrated any significant effectiveness at combating the disease.